1. Field of the Invention
This invention relates to a track guide linear motion bearing device for use as a guide element in a machine tool, apparatus or the like, and more particularly to a track guide linear motion bearing device of such a construction wherein a retainer is provided for preventing the balls of the bearing device from slipping out of position when an outer sleeve and guide bar are separated from each other.
2. Description of the Prior Art
A track guide linear motion bearing device is normally provided with an outer sleeve comprising a pillar-like member provided with an axial inner surface having a substantially C-shaped cross-section, said inner surface being alternately formed with ball rolling grooves and no-load ball guide grooves circumferentially thereof. A guide bar is also provided and typically includes in its outer surface, ball guide grooves corresponding to the ball rolling grooves of the outer sleeve. The bearing balls may roll along the endless track grooves of a retainer and are fitted between the ball rolling grooves of the outer sleeve and the ball guide grooves of the guide bar. The outer sleeve is thus rectilinearly movable relative to the guide bar and has a great capability of bearing the load because the balls are supported in concave grooves. Further advantages are that circumferential back-lash can be eliminated because the angles of contact between the balls can be made alternately different in direction, and the device is convenient to handle because the retainer prevents the balls from slipping out when the outer sleeve and the guide bar are separated from each other. However, a prior art track guide linear motion bearing device (U.S. Pat. No. 3,897,982) is such that the ball rolling grooves and the no-load ball guide grooves somewhat deeper than the ball rolling grooves are alternately formed circumferentially of the inner wall of a cylinder, and circumferential grooves having the same depth as the no-load ball guide grooves are formed in the opposite end portions thereof and, therefore, an inclined surface connecting the bottoms of the circumferential groove and the ball rolling groove is formed in the connecting portion of these grooves. Thus, the balls which have rolled down into the ball rolling grooves must suddenly change their direction of movement at this inclined surface portion, and this has led to a disadvantage that it is difficult for the balls to move smoothly.
Also, the prior art track guide linear motion bearing device is often assembled by arranging balls in the endless track groove of a retainer, and then inserting this retainer into an outer sleeve and, therefore, balls cannot be placed into the portion in which the endless track groove of the retainer overlaps the branch-off zone between the ball rolling groove and the no-load ball guide groove of the outer sleeve when the retainer is inserted and, thus, the number of balls that can be used has unavoidably been small. This in turn has led to a disadvantage that on the no-load ball guide groove side, the movement of balls is liable to become intermittent and the collision of balls becomes great to render the operation unstable.
If it is desired to incorporate a required number of balls into this prior art track guide bearing device, it is necessary to elastically deform and widen the through-hole of the endless track groove provided in the retainer and incorporate the balls from the inner surface side. This has meant a disadvantage that a very unproductive manual work is unavoidably involved to incorporate all the balls in the device.
Further, the prior art track guide linear motion bearing device is of such a construction that the retainer is guided by the balls or by the guide bar and, therefore, the balls receive resistance from the retainer, so that the rolling of the balls is not smooth, thus making it difficult to provide smooth operation.
In another prior art track guide linear motion bearing device (U.S. Pat. No. 3,938,854), the guide bar is an elongated bar-like member provided with a pair of ball guide grooves in both shoulders thereof, and provided with concave cut-away portions in the side surface portions thereof. This guide bar also has a cross-section in which the lower part of the concave cut-away portions is laterally bulged, and the ball guide groove portion of this guide bar is usually subjected to a hardening treatment to harden the surface thereof. Flame hardening or high frequency hardening is generally adopted as this hardening treatment.
Flame hardening is a advantageous in that it can be carried out relatively simply, but it is liable to produce irregularity of hardness and cause bending deformation due to the residual stress of the heat treatment. The presence of bending deformation in such an elongated guide bar would cause the finishing allowance during griding to vary greatly and would undesirably reqire a number of grinding steps to be involved.
High frequency hardening generally involves less bending deformation for bar-like elongated members and it is desirable as a heat treatment for such an elongated guide bar.
However, even by the use of high frequency hardening, it is very difficult to harden such an elongated member without bending it and, in the case of the treatment thereof, the hardening treatment is effected with the guide bar being vertically supported and while rotating the guide bar inserted in a circular high frequency coil. Even in a case where such a hardening treatment is effected, the hardness penetration is non-uniform in the upper and lower surfaces of the guide bar because the cross-sectional shape of the conventional guide bar is not symmetrical in vertical direction and, as a result, the bending deformation by the hardening has not been unavoidable, which has led to a disadvantage that a number of subsequent grinding steps are required.
Further, in the conventional guide bar, the side surfaces thereof perpendicular to the bottom surface thereof are small and, therefore, it is difficult to obtain an accurate reference surface when ball guide grooves are formed by grinding and this has led to the difficulty with which the guide bar is properly mounted on a jig when the ball guide grooves are to be formed by grinding, which in turn has made it difficult to obtain a guide bar of good accuracy.